An electron gun for use in a color cathode ray tube consists of a plurality of electrodes for focusing and accelerating the thermionic electrons emitted from a cathode, thereby forming electron beams. These electrodes each include respectively, three beam apertures through which RED, GREEN and BLUE(R, G, B) beams pass.
The electron beams projected from the electron gun toward the picture screen are deflected by the electric field generated from a deflection yoke so that the beams may be scanned toward the picture screen, thereby forming a picture.
For the pictures of the color cathode ray tube to be made clear, the size and shape of spots formed by beams striking the screen surface must be uniform at all the places thereon.
However, conventional cathode ray tubes have a basic problem that the distance between the electron gun and the edge of the screen thereof is longer than that between the electron gun and the center of the screen. Further, the irradiation angles of the beams are sharper at the edge than at the center, and therefore, the edge of the screen produces diagonally elongated beam spots, thereby reducing the picture quality.
As one method of overcoming this problem, Hsing-Yao Chen U.S. Pat. No. 4,443,736, Apr. 17, 1984, proposes a dynamic focus-type electron gun wherein auxiliary electrodes which have a vertical slot or a horizontal slot are installed respectively in front of and behind a screen electrode and which selectively apply voltage to each auxiliary electrode according to the scanning position on the screen. Whereas this kind of electron gun can form nearly uniform spots, regardless of the center and edge of the screen, it has a disadvantage that considerable complex circuit structure must be required for applying voltage or not applying voltage to each auxiliary electrode according to the scanning position of the beams.
Thus, generally, a method of forming a nearly circular spot on the edge of the screen has been used in practice. This method is to install the auxiliary electrode having an elongated beam hole at one side of the screen electrode.
FIG. 5 is an example of a commercially used screen electrode structure of an electron gun in which an auxiliary electrode 8 having elongated beam holes 6R,6G,6B is installed at the main electrode having circular beam holes 2R,2G,2B. This auxiliary electrode 8 is welded to and united with the main electrode 4. In this kind of screen electrode, voltage equal to that applied to the main electrode 4 is applied to the auxiliary electrode 8 without relation to the beam scanning position of the screen, so that all the electron beams passing therethrough are focused in the elongated beam type. As a result, whereas the elongated spot is formed on the center of the screen, the same is changed into the nearly circular spot at the edge of the screen, so that as a whole, uniform picture quality can be obtained.
Meanwhile, in the assembling process of the screen electrode having only the main electrode 4 and the auxiliary electrode 8, improper centering between beam holes 2R and 6R, 2G and 6G, 2B and 6B coaxially aligned causes an extending flare both above and below the core of the beam spot on the screen.
The conventional screen electrode as shown in FIG. 5 does not have means for the precise centering of the beam holes, so that relatively high skill in assembly is needed, yet a poor quality ratio is comparatively high.